Re: [PATCH v4 3/3] virtio-mem: disallow mapping virtio-mem memory via /dev/mem

[David Hildenbrand]

On 03.09.21 09:02, Michael S. Tsirkin wrote:
> On Thu, Sep 02, 2021 at 06:09:19PM +0200, David Hildenbrand wrote:
> > We don't want user space to be able to map virtio-mem device memory
> > directly (e.g., via /dev/mem) in order to have guarantees that in a sane
> > setup we'll never accidentially access unplugged memory within the
> > device-managed region of a virtio-mem device, just as required by the
> > virtio-spec.
> >
> > As soon as the virtio-mem driver is loaded, the device region is visible
> > in /proc/iomem via the parent device region. From that point on user space
> > is aware of the device region and we want to disallow mapping anything
> > inside that region (where we will dynamically (un)plug memory) until
> > the driver has been unloaded cleanly and e.g., another driver might take
> > over.
> >
> > By creating our parent IORESOURCE_SYSTEM_RAM resource with
> > IORESOURCE_EXCLUSIVE, we will disallow any /dev/mem access to our
> > device region until the driver was unloaded cleanly and removed the
> > parent region. This will work even though only some memory blocks are
> > actually currently added to Linux and appear as busy in the resource tree.
> >
> > So access to the region from user space is only possible
> > a) if we don't load the virtio-mem driver.
> > b) after unloading the virtio-mem driver cleanly.
> >
> > Don't build virtio-mem if access to /dev/mem cannot be restricticted --
> > if we have CONFIG_DEVMEM=y but CONFIG_STRICT_DEVMEM is not set.
> >
> > Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
>
>
> > ---
> >   drivers/virtio/Kconfig      | 1 +
> >   drivers/virtio/virtio_mem.c | 4 +++-
> >   2 files changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
> > index ce1b3f6ec325..ff80cd03f1d1 100644
> > --- a/drivers/virtio/Kconfig
> > +++ b/drivers/virtio/Kconfig
> > @@ -101,6 +101,7 @@ config VIRTIO_MEM
> >   	depends on MEMORY_HOTPLUG_SPARSE
> >   	depends on MEMORY_HOTREMOVE
> >   	depends on CONTIG_ALLOC
> > +	depends on !DEVMEM || STRICT_DEVMEM
> >   	help
> >   	 This driver provides access to virtio-mem paravirtualized memory
> >   	 devices, allowing to hotplug and hotunplug memory.
>
> It would be nicer if there was a symbol in the MEMORY_ namespace
> we can depend on exported by mm and depending on !DEVMEM ||
> STRICT_DEVMEM.
>
> E.g.
>
> config MEMORY_EXCLUSIVE
>          def_bool y
>          depends on !DEVMEM || STRICT_DEVMEM
>
> and then in virtio
> 	depends on MEMORY_EXCLUSIVE

Yes, but I'm not able to come up with an expressive name. 
MEMORY_EXCLUSIVE can be highly misleading ...


> the virtio change itself is ok though:
>
> Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>

Thanks!


--
Thanks,

David / dhildenb